I'd like to add the following indexing operations for Data.Set.Set, to complement the existing functions for Data.Map.Map:
findIndex :: Ord a => a -> Set a -> Int lookupIndex :: Ord a => a -> Set a -> Maybe Int elemAt :: Int -> Set a -> a deleteAt :: Int -> Set a -> Set a
These seem to be abstraction-breaking.
There is a natural bijection between the elements of an ordered set and their indexes in this ordering, and the new API works with the elements using the indexes instead of the elements themselves. I believe the set abstraction is not compromised by that.
And what do we expect from
let i = findIndex a s1 in deleteAt i (s1 `union` s2)
It removes the i-th least element of (s1 `union` s2). Which element is that is hard to say, depending on s1 and s2. Cheers, Milan